In vaccine clinical trials, immune responses are often used to measure the effect of a vaccine because they correlate with a vaccine’s protective efficacy against the target disease. In this talk, we will discuss the statistical issues involved in assessing immune markers and correlates of protection in vaccine clinical trials and illustrate some potential difficulties in applying classical methods (such as Prentice’s criteria) for validating surrogate endpoints. We will present approaches in evaluating correlates of protection, including the concept of protective level and the use of statistical models to examine the relationship between long-term disease protection and the whole distribution of immune responses. We will use a varicella (chickenpox) vaccine example to illustrate the methods.